What determines the shape and size of multicellular organisms is a fundamental question in biology. Plants offer a relatively simple system to study this question because they have fewer tissues and cell types than animals. The objective of this study is to understand the molecular mechanisms by which the peptide hormone Rapid Alkalinization Factor (RALF) antagonizes plant growth, specifically cell elongation. Application of exogenous RALF peptide to growing seedlings causes growth arrest of both roots and shoots, and systemic over-expression of RALF results in semi-dwarf plants. All plant genomes examined contain at least one RALF peptide-encoding gene, pointing to a fundamental role in plant growth. The molecular details of RALF-induced growth arrest are currently unknown, and understanding how this peptide hormone influences growth could allow plant biologists to manipulate plant architecture for the improvement of crops. The experiments outlined in this proposal combine physiological, genomic, and genetic approaches to understand RALF function in the model plant Arabidopsis thaliana. Specifically, the experiments are designed to understand how RALF signaling antagonizes auxin and brassinosteroid-induced growth in seedlings and to develop a transcriptional growth network by performing microarray experiments with seedlings treated with RALF1 peptide. Genetic screens have been designed to discover genes required for RALF-induced growth arrest. Receptors for plant peptide hormones described to date have been leucine rich repeat receptor-like kinases (LRR-RLKs), and we anticipate that a RALF peptide receptor would also be an LRR-RLK. LRR-RLKs in animal systems play important roles in organ development and immunity; therefore, understanding how these types of receptors function will have wide reaching implications. Together, these experiments will begin to unravel the mechanisms RALF action and add significantly to the current model of how plant cells grow.